1. Field of the Invention
The present invention concerns the generation of x-rays, and particularly time-resolved x-rays having nanosecond and shorter duration. The present invention particularly concerns x-ray sources for lithography, and especially sources providing an energetic flux of hard x-ray radiation over a spatially extended area.
2. Background of the Invention
The present invention generally relates to the production of x-ray radiation, particularly time-resolved pulses of x-ray radiation, and particularly relates to the production of x-ray radiation over a spatially extended area.
2.1 Time-Resolved X-ray Sources
The earliest attempts to produce time-resolved x-rays employed mechanical shutters that moved in front of x-ray sources. For example, transmission of x-rays through x-ray transparent apertures within a rotating apertured disk that was otherwise opaque to x-rays permitted the generation of millisecond x-ray pulses. These millisecond x-ray pulses were too slow to permit the study by x-ray diffraction of any type of molecular phenomena such as reaction, melding, dissociation, or vibration. Millisecond x-ray pulses were, however, sometimes sufficient to permit observation of certain biological phenomena, although not normally at the biomolecular level.
Davanloo et al., Rev. Sci. Instrum. 58:2103-2109 (1987) reported constructing an x-ray source capable of producing x-ray pulses of nanosecond (ns) duration. That x-ray source utilized (i) a low impedance x-ray tube, (ii) a Blumlein power source, and (iii) a commutation system for periodically applying power from the Blumlein power source to the x-ray tube. The system yielded 140-mW average power in 15 ns pulses of radiation near 1 .ANG.. That device, and others based on Blumlein-generators, suffers from (i) low repetition rates in the range of 100 hertz, (ii) prospective inability to produce pulses shorter than about 15 nsec, and (iii) low energy efficiency on the order of 25%. The durability in operational use of Blumlein-based sources of x-ray flashes is also uncertain.
More recently, Science News, Vol. 134, No. 2: pp. 20 (1989) reported that scientists at Cornell University and the Argonne National Laboratory have developed a device, called an undulator, capable of producing x-ray pulses one-tenth of a billionth of a second (100 picoseconds) in duration. The undulator utilized synchrotron radiation from fast-moving charged particles in an electron storage ring. Because electron storage rings are typically large and expensive, the ring used at Cornell being one half-mile in diameter, the production of bright x-ray flashes by such means is distinctly not adaptable to the scale and budget of a typical materials or biological laboratory.
X-rays have been produced using plasma sources that are energized by lasers. In laser plasma x-ray sources, either a pulsed-infrared (IR) laser or a ultraviolet (UV) excimer laser is used with pulse widths varying from less than 10 picoseconds to 10 nanoseconds. The laser beam is focused on a target where it creates a plasma having a sufficiently high temperature to produce continuous and characteristic x-ray radiation. Major disadvantages of laser plasma x-ray sources include (i) a diffuse, non-point, area of x-ray emission (ii) low efficiency (iii) low repetition rate.
2.2 X-ray Sources for Lithography
Since the seminal paper by Henry Smith appeared in 1972, the achievement of economical x-ray lithography has been rather elusive. During the intervening years, however, considerable progress in many areas has been made, including development of masks, resists and registration capabilities.
Three main classes of x-ray sources are considered as a possible choice for lithography. Those are electron impact tubes, laser-based plasmas, and synchrotrons. Progress has been made in each of these sources, particularly in laser-driven plasma x-ray sources. Efforts in Japan have been devoted to the development of compact, high density synchrotrons. Even today, each of these sources has its limitations for a practical system.
The most intense sources are the synchrotrons, but so far their price, size and complexity make them prohibitive for use in a production line.
Electron impact tubes are the simplest and cheapest sources. However, their effectiveness is best only in the hard x-ray region. For high current output electron impact tubes must be pulsed because of the extreme heat generated on the anode by electron impact on the anode.
Laser driven x-ray sources have started to appear and show promise.
The requirements for a practical x-ray source for lithography are dependent on development of the other two critical components of the lithographic process--mask and resist. Most of the research and development for x-ray sources is centered in the 0.4-5 nm wavelength range where suitable resists are available. Use of still harder x-rays, 0.1-1.0 nm, would bring additional benefits, such as the possibility of ultrasensitive microsensors for medical and technological applications and, of course, higher resolution lithography permitting a denser layout of semiconductor components.
The present invention will be seen to be concerned with the generation of x-ray pulses for lithography in a manner that is believed to provide several distinct advantages over previous x-ray sources.
2.3 Photoemissive Sources of Electrons
By way of background to the present invention, Lee, et al., in Rev. Sci. Instrum., 56:560-562 (1985) described a laser-activated photoemissive source of electrons. In the laser-activated photoemissive electron source a photocathode is illuminated with high intensity laser light as a means of generating numerous electrons by the photoelectric effect. The electrons emitted from the photocathode are focused in an electrical field, typically produced by electrodes in an electron-gun configuration, in order to produce a high intensity electron beam.
2.4 Rectification of Ultrashort Optical Pulses to Produce Electrical Pulses
By way of further background to the present invention, the rectification of ultrashort optical pulses in order to generate electrical pulses having durations and amplitudes that are unobtainable by conventional electronic techniques is described by Auston, et al, in the Annl. Phys. Lett., 20:398-399 (1972). Electrical pulses on the order of 4 amperes in 10 picoseconds are generated by rectification of 1.06 micrometer optical pulses in a LiTaO.sub.3 crystal doped with approximately 2.24% Cu (LiTaO.sub.3 :Cu.sup.++).
A doped transmission line, having an absorption coefficient of 60 cm.sup.-1 and a thickness of 0.2 mm, is bonded with a thin epoxy layer to an undoped crystal in the form of a TEM electro-optic transmission line of 0.5.times.0.5-mm cross-sectional area. Current pulses are generated by absorption in this transducer of single 1.06 micrometer mode-locked Nd: glass laser pulses, typically of duration 3-15 psec and with an energy of approximately 1 mJ.
The electro-optic transmission line, or switch, operates to conduct current during the presence of laser excitation by action of the macroscopic polarization resulting from the difference in dipole moment between the ground and excited states of absorbing Cu.sup.++ impurities. Effectively, the electric-optic transmission line, or switch, has a very great number of charge carriers, and is a very good conductor, during the presence of laser excitation. During other times it is a semiconductor and does not conduct appreciable current. The excited-state dipole effect of the transmission line, or switch, is exceptionally fast, on the order of 1 or 2 psec or less.